Designer phospholipid capping ligands for soft metal halide nanocrystals

The success of colloidal semiconductor nanocrystals (NCs) in science and optoelectronics is inextricable from their surfaces. The functionalization of lead halide perovskite NCs1–5 poses a formidable challenge because of their structural lability, unlike the well-established covalent ligand capping of conventional semiconductor NCs6,7. We posited that the vast and facile molecular engineering of phospholipids as zwitterionic surfactants can deliver highly customized surface chemistries for metal halide NCs. Molecular dynamics simulations implied that ligand–NC surface affinity is primarily governed by the structure of the zwitterionic head group, particularly by the geometric fitness of the anionic and cationic moieties into the surface lattice sites, as corroborated by the nuclear magnetic resonance and Fourier-transform infrared spectroscopy data. Lattice-matched primary-ammonium phospholipids enhance the structural and colloidal integrity of hybrid organic–inorganic lead halide perovskites (FAPbBr3 and MAPbBr3 (FA, formamidinium; MA, methylammonium)) and lead-free metal halide NCs. The molecular structure of the organic ligand tail governs the long-term colloidal stability and compatibility with solvents of diverse polarity, from hydrocarbons to acetone and alcohols. These NCs exhibit photoluminescence quantum yield of more than 96% in solution and solids and minimal photoluminescence intermittency at the single particle level with an average ON fraction as high as 94%, as well as bright and high-purity (about 95%) single-photon emission.


Supplementary Note 1. Slab model of perovskite surfaces
The surface of perovskite NCs was modelled using a crystal slab with a size of 8  8  3.5 primitive unit cells.The primitive unit cell was chosen as a basis for crystallographic description to allow direct comparison between different perovskite structures.In the case of room-temperature orthorhombic modification of CsPbBr3 1 , it is characterized by two vectors of equal length   =   with the angle  = 89.666° between them, and another orthogonal vector   .Therefore, two nonequivalent (001) and (010) surfaces were considered for CsPbBr3 that correspond to the same (100) surface of cubic FAPbBr3.The simulation box was chosen according to the symmetry of the perovskite crystal, e.g., rectangular box was used for FAPbBr3 and monoclinic boxfor CsPbBr3.The simulation box was always kept fixed in XY plane according to the experimental lattice parameters, whereas box length along Z axis was either fixed or allowed to change, depending on the simulated thermodynamic ensemble.A typical value of the box length after equilibration was about 6.5 nm.
Different stoichiometries of the perovskite surfaces were prepared and studied with replica-exchange MD simulations (Supplementary Fig. 1).Symmetric PbBr-terminated slab was always used for [ABr] = 0, and ABr-terminated slab for [ABr] = 0.5 and 1.

Supplementary Note 2. Details of the replica-exchange MD simulations
Positions of all ions except the top-most surface layer were restrained with a flat-bottomed harmonic potential (k = 5000 kJ/(mol•nm 2 )) to a spherical volume with a radius corresponding to 1/4 of the unit cell size to avoid melting of the entire crystal and to prevent diffusive movement of the ions at high temperatures.
The corresponding region of the slab is referred to as "frozen" in Supplementary Fig. 1.We note that, despite the name, these restraints have negligible effect on lattice vibrations and distortions at 300 K (Supplementary Fig. 2).Movements of the ligands and the surface ions were limited in Z direction by adding repulsive walls at 3 nm above and 0.3 nm below the surface to prevent their migration to the opposite side of the slab.
Our computational setup was found to have an artifact in the case of the orthorhombic CsPbBr3 slab, which is observed as a spontaneous change of its crystallographic orientation.The artifact is caused by relatively small free energy differences between alternative crystallographic orientations of the slab, whereas the corresponding barriers can be easily overcome in replica-exchange MD simulations.To favor the desired orientation, additional harmonic restraints (k = 150 kJ/(mol•rad)) were applied to a set of three orthogonal Br-Br-Br angles (plus three neighboring angles that are parallel to them) formed by distant to the ligand corner-sharing PbBr6 octahedra in the middle layer of the slab (Supplementary Fig. 3).
To further limit the configurational space that has to be sampled, a different set of restraints was used for systems containing a single ligand molecule.Firstly, only the minimum number of surface ions were allowed to exchange with the ligand, whereas all the other ions were restrained with the aforementioned flatbottomed potentials.Second, movements of these ions and the ligand molecule were limited in XY plane in order to keep them close to the place of interest.Details of the restraining scheme are summarized in Supplementary Fig. 4. We note that this restraining scheme, while not eliminating any of the physically relevant binding modes completely, introduces some bias into their populations, which needs to be removed by reweighting.Qualitatively, reweighting factors follow the trend BM1 < BM2 = BM2' < BM3, which means that the actual populations of the BM3 (see Fig. 2a and Extended Data Fig. 2) should be even closer to unity.Supplementary Fig. 4 | Restraints that were used in replica-exchange MD simulations of single ligand molecules at the FAPbBr3 (a) and CsPbBr3 (b) surfaces.Blue and red filled circles denote ions which were allowed to exchange with the ligands.Movements of ions and ligand head-groups (N and P atoms) were limited in XY plane to a square, which is denoted by the blue line.The size of this square was chosen in such way that it accommodates ligand head-group in its fully extended conformation (rextended), independent of its attachment point at the surface.

Supplementary Note 3. Assessing ligand binding modes on NC surfaces via FTIR spectroscopy and ab-initio molecular dynamics simulations at the DFT level
Ab-initio molecular dynamics simulations at the density-functional level of theory were performed using the CP2K computational package (version 8.2), 3 employing a PBE exchange-correlation functional.Core electrons were described using effective core potentials and outer electrons with a double-ζ basis set augmented with polarization functions (DZVP).Starting from a charge-neutral FAPbBr3 NC with a size of 3x3x3 unit cells and the same surface termination as in our classical molecular dynamics simulation, we add either six PEA or six PC ligands with a density of one ligand per facet (placed in the center of the facet).
After an initial geometry optimization and subsequent equilibration of the total energy at 300 K (NVT ensemble with a 1 fs time step and a CSVR thermostat, i.e., canonical sampling via velocity rescaling), 4 we obtain a nuclear trajectory at 300 K of about 3 ps within the NVE ensemble utilizing a 1 fs time step.The latter trajectory is provided to the TRAVIS (Trajectory Analyzer and Visualizer) computational package, which computes the phonon density of states (DOS; power spectrum) via Fourier transformation of the mass-weighted autocorrelation of the nuclear positions along the trajectory.The spectrum of certain vibrations of interest, e.g., those involving the PO4 -and NH3 + ligand functional group, can be extracted via suitable sub-selection of atom groups within the entire structure.While the thus obtained phonon DOS still lacks information on the expected IR intensities, the good agreement of such AIMD-derived mode frequencies with the experimental IR frequencies (typically within ~10 cm -1 ) increases the accuracy of our vibrational mode assignments, especially in case of spectrally congested regions and/or lacking or widely varying literature reports.
Supplementary Fig. 9a-b discusses major changes in bonding expected when PC and PEA ligands are coordinating to the perovskite surface in BM3.The difference between the two molecules is the ability of PEA headgroup to form close PO…HN contacts through H-bonding due to its strongly donating NH3 + group, while the N(CH3)3 + headgroup of PC is screened from these interaction. 5Once the headgroup coordinates to the NC surface, a significant change in H-bonding of the NH3 + group is expected.
We analyze and compare experimental FTIR spectra of neat PC and PEA ligands with corresponding spectra of CsPbBr3 and FAPbBr3 NCs capped with these ligands (Supplementary Fig. 9c-d).The main region to reveal differences between the neat ligand and NC-surface-bound ligand is between 700 and 1800 cm -1 .A prominent change occurs in the P-O stretching region, 6 from about 1050 cm -1 to about 1300 cm -1 .7][8][9] In the case of the PEA ligand (Supplementary Fig. 9d), a clear split of as(PO2) indicates strong hydrogen bonding PO2 -…H-NH2 + .The symmetric PO2 -stretch occurs at 1085-1100 cm -1 , but is harder to interpret due to overlap with, amongst others, the C-N stretching (940-1100 cm -1 ).The asymmetric PO2 -stretch is also present in the FTIR spectra of the PC-and PEA-capped CsPbBr3 and FAPbBr3 NCs and it displays a significant bathochromic shift to around 1180 cm -1 for PC and 1200 cm -1 for PEA.The region towards 1100 cm -1 is called 'ionic phosphate vibration' in the literature. 10A shift towards this region is characteristic of a weakening of P-O bonding due to an interaction with cationic species.The electronegativity of the cationic species is also reflected in the magnitude of the shift: e.g., in silver salt of di-n-butyl phosphate, as(PO2) vibration occurs at 1152 cm -1 , while in the corresponding lead salt, the same peak occurs at 1184 cm -1 . 10Examples even closer to PC and PEA are calcium salts of phosphorylcholine and aminoethylphosphate with as(PO2) at 1140-1150 cm -1 . 11,12The as(PO2) shift in PC and PEA capped NCs agrees with literature and can therefore be unambiguously interpreted as dissolution of PO2 -…H-NH2 + hydrogen bonding and phosphate binding to cationic species with subsequent P-O bond weakening and elongation.
To further identify the binding mode and distinguish between the modes BM1, BM2, BM2' and BM3 discussed in the main text, we have performed ab-initio molecular dynamics (AIMD) calculations of the phonon spectra for each of these four cases.Supplementary Fig. 10 represents the results for PEA ligands placed on a FAPbBr3 NC.There are two major possibilities for the phosphate group to engage in binding on the perovskite NC surface: bound to Pb (in BM3 or BM2', Supplementary Fig. 10a) or unbound (BM1 or BM2).In the unbound case, if the positive zwitterionic group (NH3 + or N(CH3)3 + ) substitutes the perovskite A-site, the negative phosphate group must be neutralized, likely with an A + (FA + or Cs + ) cation (BM2, Supplementary Fig. 10b).To distinguish phosphate from being either bound to Pb (BM3 or BM2') or unbound (BM2 or BM1), we analyze the PO4 contribution to the total calculated phonon power spectrum (Supplementary Fig. 10c).In both cases of Pb-bound phosphate (BM2' and BM3), we find the contribution from as(PO2) at around 1180 cm -1 , whereas for a FA + neutralized unbound phosphate (BM2), this vibrational transition is observed at around 1140 cm -1 (and with decreased intensity).The closer agreement of the as(PO2) peak location for BM3 and BM2' with the corresponding frequencies observed in the experimental FTIR spectrum of PEA-capped FAPbBr3 suggests that PC and PEA engage in either BM2' or BM3, the latter being consistent with the prediction from the replica-exchange MD simulations presented in the main text.
To further distinguish between BM3 and BM2, we further investigate vibrations involving the cationic species of the zwitterion headgroup in PEA and PC, i.e., NH3 + and N(CH3)3 + , respectively.7][8][9] For PC-capped CsPbBr3 and FAPbBr3 NCs, these peaks show a significant intensity decrease, broadening, and a very small low-frequency shift (Supplementary Fig. 11a).Additionally, there is a small bathochromic shift of the CH3 of the trimethyl ammonium headgroup asymmetric bending modes (as (N-CH3)) in the C-H bend region around 1480 cm -1 (Supplementary Fig. 11b).The C-H bend region, however, is ambiguous due to contributions from other CH3 and CH2 entities of the ligand tail, rendering these changes ill-suited for interpretation.
7][8][9] Additional confirmation for the assignment of the NH3 + vibrations is drawn from the comparison to ND3 + in PEA-d3 (Supplementary Fig. 12).Splitting of the as(NH3) at 1643 cm -1 indicates hydrogen bonding between ammonium and phosphate groups.Upon binding to the NCs, both s(NH3) and as(NH3) show strong bathochromic shifts to around 1500 cm -1 and 1600 cm -1 , respectively, which is consistent with the major loss of hydrogen bonding that occurs between NH3 and PO4 groups in the neat ligand.To interpret these shifts, a useful comparison is the N-H bending from the methylammonium cation, CH3NH3 + , in bulk CH3NH3PbBr3: s(NH3) and as(NH3) appear at 1477 cm -1 and 1585 cm -1 , respectively, 13 which is rather comparable with the shift found in PEAcapped perovskite NCs.AIMD simulations of the phonon spectra for all binding modes (Supplementary Fig. 10c) reveals s(NH3) and as(NH3) signals for BM2 and BM3 (each with the NH3 + group substituting the A cation) at the same location as in the experimental spectrum, around 1500 and 1580-1600 cm -1 , as expected for BM3.
In summary, the combined FTIR findings on phosphate and ammonium headgroups, corroborated by AIMD simulations and comparison to prior literature, conclude that only BM3 is holds for PEA-capped FAPbBr3 and CsPbBr3 NCs.BM3 is a prevailing binding motif of the PEA-ligand binding also in our computational prediction of the ligand binding (replica-exchange MD simulations, Fig. 2) and REDOR NMR data.For the PC ligand, while the phosphate FTIR shifts (and REDOR NMR data) attest the binding of the phosphate to the Pb atoms, we refrain from strong assertions as to the N(CH3)3 + group insertion into the A-site owing to spectral overlaps and lack of hydrogen-bonding capability (so instrumental for PEA).

2-Ammonioethyl
TOP-Br2 precursor solution was prepared by mixing TOP (6 mL, 13 mmol, 1 eq) with Br2 (0.6 mL, 11.5 mmol, 0.88 eq).The reaction between the two components is exothermic and requires vigorous stirring due to the product being highly viscous (white, almost solid).Mesitylene (18.7 mL) as added to the reaction, yielding a 0.46M light-yellow stock solution.The reaction was carried out in a Schlenk flask under Argon.
In a typical FAPbBr3 NCs synthesis, 1 eq. of FAOA (1 ml, 0.2M) and 1.6 eq. of Pb(OA)2 (0.5 ml 0.4M) precursors were combined with 5 ml of mesitylene along with 0.25 eq. of PC ligand or 0.25 eq. of PEA ligand of choice (added as dry solids).The reaction vessel was heated to required temperature (typically 70C for PC and 90C for PEA) and TOP-Br2 (0.5 ml) was injected.The reaction was cooled with ice bath immediately, yielding a bright green colloid (Supplementary Fig. 36b).NCs were purified by adding 1-2 eq. of acetonitrile, centrifugated and redispersed in a solvent of choice (e.g., hexane for C8C12-PEA).The purification procedure was repeated up to 5 times to remove precursors and unbound ligands, resulting in NCs colloids capped with PC and PEA ligands (Supplementary Fig. 36c-d).PEA-capped FAPbBr3 NCs retain colloidal stability upon extended storage, while PC-capped NCs precipitate and sinter over couple days.

Fig. 1 |
Preparation of perovskite surfaces with different stoichiometry.a, Steps used to construct input structures.b, Example of the surface with [Lig] = 50% and [ABr] = 50% (top view).Note that the initial positions of ligands and vacancies in the surface ABr layer do not match.c, Example of the surface with [Lig] = 50% and [ABr] = 0% (top view).In both cases only two surface layers are shown.d,e, Side views of the corresponding slabs.Regions which are kept "frozen" in replica-exchange MD simulations are marked with dashed rectangles.

Fig. 5 |
NCs surface termination.a,d, High resolution HAADF-STEM images of FAPbBr3 (a) and CsPbBr3 (d) NCs capped with PEA ligands.b,c,e,f, (100) lattice spacing of FAPbBr3 (b) and CsPbBr3 (e) NCs and the corresponding FFTs (c,f).Supplementary Fig. 6 | Validation of the perovskite model.a, Comparison between the experimentally derived structural model (left) and MD simulation snapshot (right) of FAPbBr3 at 300K.In agreement with the experiment, cubic perovskite structure with disordered FA orientations is observed.b, Comparison between the experimentally derived structural model (left) and MD simulation snapshot (right) of CsPbBr3 at 300K.In line with the experiment, orthorhombic perovskite structure is observed.Predicted lattice parameters agree well with the experimental data.

2 *Supplementary Fig. 7 |
"A" denotes the center of a cationic groupnitrogen atom of the PC ligand, center of mass of the -CH2NH3 + group in PEA, or crystallographic position of the surface A site, correspondingly.Similarly, "X" denotes the center of an anionic groupphosphorus atom of the PC or PEA ligand, or crystallographic position of the surface X site.Angle brackets indicate trajectory averages, and   indicates a projection along the surface normal.Details of the REDOR experiment.The dipole-dipole interaction is at the foundation of a REDOR NMR.A shorter dipole-dipole distance induces a faster dipole decay, resulting in a steeper S/S0 REDOR curve.Supplementary Fig. 8 | 31 P 207 Pb REDOR NMR results for NCs capped with oleyl PC and PEA ligands at different magnetic fields.a-b, PC-capped NCs at 600 MHz (a) and 400 MHz (b).c-d, PEA-capped NCs at 600 MHz (c) and 400 MHz (d).

Supplementary Fig. 9 |Supplementary Table 3 .
Infrared spectroscopy analysis of the bound and free ligand vibrations.a-b, Differences between neat PC (a) and PEA (b) ligands and ligands bound to a perovskite surface in BM3.A distinct difference between PC and PEA molecules is a much stronger intermolecular hydrogen bonding in the latter.Upon binding to perovskite, the main change is expected for the PO4 -group (bound to Pb in BM3) and the NH3 + group of PEA (dissolution of a strong NH3 + -PO4 -H-bonding).c-d, Experimental FTIR spectra of the PC (c) and PEA (d) ligands in the free (neat) form, as well as after binding to FAPbBr3 and CsPbBr3 NCs.Major bathochromic shifts are observed for the PO4 -group of both PC and PEA (PO2 symmetric and asymmetric stretch, 1150-1300 cm -1 ) and the NH3 + group of PEA (NH3 symmetric and asymmetric bend, 1500-1650 cm -1 ).A minor bathochromic shift is observed for the CH3 asymmetric bend from N-methyl groups in PC (1475 cm -1 ).General differences between bulk and NCs surface bound ligands also include peak broadening.Summary of the relevant FTIR peaks for the PC ligand.